Poor Allison, locked in the cupboard while Kayako/Aubreysloooowly comes for her. Allison's terrified, sobbing screams make the whole thing so much worse.
Kayako emerging from one of Grethen's paintings, and, prior to that, the horribly unsettling damage she does to the painting of Rose.
Naoko's unsettling walk and horrible gurgling sounds after she too becomes a Stringy-Haired Ghost Girl... not to mention her (presumed) prime method of killing.
Not to mention Naoko's (horribly brutal) death in the first place. To elaborate: she has her knee broken and a serrated knife forced through her throat.Yikes.
Yoko in the attic. She switches on her lighter and pokes her head into the attic, slowly looking about her all the while hearing what are seemingly odd creaking noises (of course, as those who are familiar with the original Ju-on films will know, those noises are certainly not caused by anything creaking). As she turns, a pale face can suddenly just be made out behind her... and then her lighter highlights this mysterious face.
Speaking of Yoko: There's a part where Alex sees her walking very stiffly after she's been missing for days. She turns around upon being called - and her entire jaw is missing, complete with her tongue lolling out because there's no place to hold it. Eep.
The Grudge 3 has a rather frightening sequence of Kayako chasing Dr. Sullivan through the hospital but no matter where she turns. Kayako is right on her. She mananges to flee until coming to a door that won't open to her card and screaming for a janitor to help her. All the while we see Kayako warping to her, not even walking. She even phases through a closed door to get to the room Sullivan's in before she grabs her neck and snaps it.
Susan being stalked from her office to her own bedroom - nowhere is safe.
Aubrey's death from the second film. The Sickening "Crunch!" and The Scream she emits just before her neck is snapped is cringeworthy. Also is you listen close to her death rattle you can actually hear her wheezing for air, which makes this situation much more frightening and tear jerking.
There's also the disturbing aspect that Aubrey is basically lying there, dying and then sees Kayako peak her head through the door, open it and slowly approach her. There's something so unnerving of seeing Kayako not only looking normal for once, but just the surreal nature of it. And then we see her creepy glare..
The hand appearing in Karen's hair in the first film - it was basically the money shot used in the trailer to get people to see the film.